Conventionally, as is shown in Patent Document 1, an FTIR (Fourier-transform infrared spectroscopy) method is used in order to measure concentrations of measurement target components contained in a test sample such as, for example, exhaust gas or the like.
A spectrophotometer that uses this FTIR method is provided with a measurement cell into which a test sample is introduced, a light irradiation unit that irradiates infrared light onto the measurement cell, and a photodetector that detects an intensity of light transmitted through the measurement cell. This spectrophotometer calculates an optical absorption spectrum of exhaust gas using light intensity signals obtained by the photodetector, and calculates the concentration of the measurement target component from the absorbance of this optical absorption spectrum. Here, when calculating a concentration from the absorbance of the optical absorption spectrum, a calibration curve comparing the absorbance of the optical absorption spectrum with the concentration of the measurement target component shown by this absorbance is used.
In the above-described spectrophotometer, during the measurement of a test sample, the temperature of the measurement cell is adjusted so that it remains constant. If the temperature of the measurement cell differs from the temperature of the measurement cell at the time when the calibration curve was created, then discrepancies occur in the concentrations obtained from the calibration curve.
As is shown in Patent Document 2, a spectrophotometer that corrects the calibration curve at each temperature has been developed in order to reduce discrepancies caused by these temperature changes. More specifically, using the same sample, this spectrophotometer measures a spectrum in advance at a reference temperature and at a different temperature from the reference temperature, and thereby determines a difference spectrum thereof. By then modifying the difference spectrum that has been multiplied by a coefficient in accordance with the temperature changes in the sample such that it is set to the measurement spectrum, the difference spectrum is converted into an equivalent spectrum to the spectrum measured at the reference temperature. The spectrophotometer then corrects the calibration curve calculation results obtained from temperature changes in the sample.